The present invention relates generally to providing dimming control signals for a fluorescent ballast. More particularly, this invention pertains to encoding dimming signals for a ballast over a power line to reduce potential unintended dimming effects of noise.
Referring to FIG. 1, a single-burst signaling system 100 is shown as previously known in the art. This system uses a high frequency burst 102 of approximately 9.8 kHz transmitted coincident with the zero crossings 104 of the AC mains 106. The presence or absence of the burst within a group of eight zero crossings 108 (or one frame 108) represents the logical state of each bit 110. In the example shown, a single burst within a group of zero crossings, or set, decodes as “0”. No bursts detected within a set decodes as “1”. The bits are organized into repeating bit patterns whose one-to-zero density is proportional to the desired dimming level. The repetitive bit patterns are encoded such that ones and zeros are proportionally distributed, allowing larger groups of bits 112 (super-frames 112) to be evaluated anywhere within the repeating pattern.
The single-burst signaling system unfortunately does not adequately accommodate more than one control channel. It is desirable in many modern ballasts to provide, in addition to a dimming level control signals as above, remote capability for sending commissioning messages, tuning control of the maximum and/or minimum light output for a ballast, and daylight harvesting signals to automatically adjust the light output in response to changes in the ambient light level in a particular area. While multiplexing of signals from more than one source along a common communications line is known to those of skill in the art, a single-burst signaling system cannot distinguish between various sources along the line.
Various systems are further known to those of skill in the art for generating and detecting pulses carrying ballast control information over a power line. However, in some of these systems detection of the transmitted pulses is difficult where the pulse width changes in response to changes in the input voltage. Other systems have the disadvantage of causing unintentional lamp dimming in the presence of high levels of power line noise about the control signals transmitted to the ballast.
What is needed, then, is a system for generating and detecting control signals over a power line without being susceptible to unintentional lamp dimming as a result of power line noise.
It is further desirable to provide a system for generating and transmitting control signals from multiple control sources over a common power line to a ballast configured to detect the signals.